The invention relates more particularly to an electrostatic print head by means of which an electrostatic latent image is progressively built up on the recording medium by ion discharge obtained by raising the electrodes to high tension as the recording medium is moved past the head.
If good resolution is required, the print head must have a very large number of very small electrodes, for example when printing at eight points per millimeter on a recording medium in A4 format (21 cm or 8.2" wide) then 1728 printing electrodes are required.
In practical implementations, the electrodes are associated with counterelectrodes raised to a high tension of opposite polarity in order to avoid applying the full high tension required for setting up the ion discharge necessary for printing an electrostatic image solely to the said electrodes. Under such conditions, the high tension necessary for obtaining ion discharge is applied across a corresponding electrode/ counterelectrode pair with the absolute value of the high tension applied solely to the electrode or solely to the counterelectrode being less than a threshold value for said ion discharge, and thus being incapable of giving rise to printing.
When performing indirect electrostatic printing, an intermediate recording medium such as a thin insulating film stored in roll form is used to receive the electrostatic latent image, and the latent image is then developed, for example by using a magnetic brush to transfer particles of ink thereto. The developed electrostatic image is then transferred from the intermediate medium to a final medium (generally paper in the form of separate or separable sheets) for example by direct contact between the intermediate medium and the final medium while applying high pressure.
When performing direct electrostatic printing, the intermediate medium is omitted, but the final medium must be constituted by a conductive base which is coated with a thin dielectric deposit enabling the latent image to be formed thereon, enabling the image to be developed by means of ink powder, and then enabling the developed image to be fixed, all on the same medium.
Published european patent specification No. 0 124 856 describes an electrostatic print head comoprising firstly at least one row of individual electrodes disposed at regular pitch and organized in n groups, with said groups being themselves organized in successive sets of at least two groups per set, and with electrodes occupying the same position within a group and the same group within a set being interconnected. The above-mentioned electrostatic print head further comprises a counterelectrode facing said electrodes over not less the length of the row under consideration and constituted by a resistive material which is in contact with n+1 conductors disposed at regular intervals at substantially the same pitch as the groups of electrodes within the row. The n+1 conductors define therebetween n resistive portions having substantially identical resistances and connected in series, with each of said resistive portions being disposed opposite a corresponding one of the groups of electrodes and being attributed to selecting said group from the other groups.
In the embodiment shown in said European patent specification, a very large number of connections need to be provided both for the electrodes and for the counterelectrodes since there are 73 counterelectrodes, for example, for a particular print head embodiment which includes 1728 points.
The present invention thus proposes firstly an arrangement of counterelectrodes enabling a portion of the counterelectrode control electronics to be incorporated in the counterelectrode itself so as to reduce the number of connections.
In addition, the movement of the, or each, print medium between the electrodes and the counterelectrodes wears down said electrodes and/or counterelectrodes and uselessly brakes the movement of the medium.
The present invention thus also proposes a counterelectrode arrangement of the kind mentioned above which seeks to reduce braking on the medium and wear on the counterelectrodes.